The DNA-transfer method has recently been shown to be an effective means of recovering SV40 and Rous sarcoma virus from virus-free transformed cells. It is proposed to define in detail basic parameters of the technique, especially the effect of size and uptake of the transformed cell DNA on virus rescue as an aid to designing future rescue attempts. Such information would also be relevant to a host of gene-transfer experiments in mammalian cells based on the uptake of nonviral nucleic acids. It is also proposed to study the mechanism of virus rescue, a working hypothesis being that the integrated SV40 genome is excised from the transformed cell DNA following its introduction into simian cells. The delineation of an SV40 excision system and the demonstration of its presence or absence in transformed cells would be informative with respect to control mechanisms operative in transformed cells. It is also proposed to adapt the DNA-transfer method to the adenovirus and defective adenovirus-SV40 hybrid virus (PARA) systems. Infectious virus has never been recovered from those transformed cells; if that were accomplished, it would provide direct evidence that the complete viral genome is present and would suggest that it is functionally repressed.